There is a continuing trend in the semiconductor industry to fabricate integrated circuits of increasing complexity. The fabrication of extremely complex, high-density integrated circuits has been made possible through advances in integrated circuit fabrication technology. Fabrication technology now exists having the capability to define circuit components having feature sizes in the sub-micron size range. For example, new lithographic techniques have been developed using x-ray and pulse-laser energy sources. Additionally, film deposition technology now exists with the capability to form thin-films having precisely determined metallurgical compositions and thicknesses. Furthermore, film deposition techniques have been developed which are capable of directionally depositing metal films in precisely defined locations during device fabrication.
To maintain a small die size, high-density integrated circuits are now commonly fabricated with multiple levels of metal interconnects. Typically, the metal interconnect layers are separated by interlevel dielectric (ILD) layers and electrically coupled by metal-filled vias residing at selected locations in the ILD layers. The vias are filled by a metal plug which provides an electrical conduit between stacked metal interconnect layers. Typically, the metal plug is a refractory metal such as tungsten.
Tungsten has become an increasingly popular material for the formation of metallized via plugs. Tungsten possesses high electrical conductivity and can be readily deposited into high-aspect-ratio vias. Additionally, a variety of deposition processes are available for the formation of both blanket tungsten and selectively deposited tungsten. However, the physical properties of tungsten differ substantially from the physical properties of aluminum and aluminum alloys. Tungsten is a mechanically hard, high-density metal having a high melting point. In contrast, aluminum is a soft, ductile metal having a relatively low melting point, and is further characterized by a large grain structure. When electrons flow from hard metals, such as tungsten, to a softer metal, such as aluminum, a flux divergence occurs at the metallic interface. In the softer metal, the flow of electrons pushes metal atoms in the softer metal away from the interface. This phenomenon is known as electromigration and severely degrades the current handling capability of the metallization structure. Because tungsten has a high density and melting point, it does not experience a mechanical deformation when subjected to a high electrical current. Moreover, tungsten does not self-diffuse when subjected to high electrical current. Therefore, electromigration of the softer metal, in the direction of electron flow, causes a void at the tungsten-aluminum interface.
In view of the mechanical deformation characteristics associated with the use tungsten filled vias, there is a renewed interest in the fabrication of metal interconnect structures using aluminum and aluminum alloys exclusively. However, even with the use of aluminum and its alloys, highly reliable interconnect structures require the use of a dopant, such as copper, and the like, to resist void formation caused by electromigration in the metallized interconnect. While chemical vapor deposition techniques using metal organic precursors have expanded the use of chemical vapor deposition as a method for depositing an aluminum and aluminum alloy, the deposition of a uniformly doped metal in a high aspect-ratio via is difficult to obtain. Additionally, conventional metal sputtering does not provide a conformally deposited metal layer, such that all surfaces of a high-aspect-ratio via can be completely covered with metal. Accordingly, an improved process is necessary for the fabrication of a reliable metallized interconnect structure in high aspect-ratio via openings.